What is DESQview/X? Many people, in the current day and age, may have never even heard of this system from the mid-1990s.
Its predecessor, DESQview (without the “/X”) which was first released in 1985, was a multi-tasking, windowing system for DOS. It allowed someone, with very modest PC hardware, to run multiple text-mode DOS applications at the same time. With overlapping, resizable windows. DESQview had true, preemptive multi-tasking. Fast. Stable. Lightweight, Impressive.
Then DESQview/X came along, in the 1990s, bringing a complete X11 (aka X Windows) graphical interface with it.
Many might make the case that DESQview/X is not actually an Operating System. It does, after all, sit on top of MSDOS (much like early versions of Windows).
A preemptive multitasking system, capable of running MSDOS, Windows, local X11, and remote X11 applications? That’s more power than many other Operating Systems even of today. So, yes. DESQview/X is an OS.
Read much more on this classic software on The Lunduke Journal of Technology.